22 CHEMISTRY OF PLANT LIFE 



of the water and carbon dioxide and the resultant synthetic process, 

 must be returned to the air by a gas exchange. 



Of the five steps in this process, the first two and the last are 

 essentially purely physical phenomena, the chemical changes 

 involved being those of the third and fourth steps. Hence, it is 

 only these two parts of the process which need be taken into 

 account in a consideration of the chemistry of photosynthesis. 



FORMALDEHYDE, THE SIMPLEST CARBOHYDRATE STRUCTURE 



The simplest carbohydrates known to occur commonly in 

 plant tissues are the hexoses (see Chapter IV) having the formula 

 CeHi 2 O6, which is just six times that of formaldehyde, CH 2 O. 

 Also, it is known that formaldehyde easily, and even sponta- 

 neously, polymerizes into more complex forms having the general 

 formula (CH 2 O)n; trioxymethylene, CsHeOs, being a well-known 

 example. Further, both trioxymethylene and formaldehyde 

 itself can easily be condensed into hexoses, by simple treatment 

 with lime water as a catalytic agent. Hence, it is commonly 

 believed that formaldehyde is the first synthetic product resulting 

 from photosynthesis, that this is immediately condensed into 

 hexose sugars, and that these in turn are united into the more 

 complex carbohydrate groups which are commonly found in plants 

 (see Chapter IV). 



There is considerable experimental confirmation of the sound- 

 ness of this view. The whole photosynthetic process takes place 

 in chlorophyll-containing plant tissues with astonishing rapidity, 

 sugars, and even starch, appearing in the tissues almost imme- 

 diately after their exposure to light in the presence of carbon 

 dioxide. Hence, any intermediate product, such as formaldehyde, 

 is present in the cell for only very brief periods and in very small 

 amounts. But small amounts of formaldehyde can often be 

 detected in fresh green plant tissues and, as will be pointed out 

 below, the whole process of photosynthesis, proceeding through 

 formaldehyde as an intermediate product, can be successfully 

 duplicated in vitro in the laboratory. 



Assuming, then, that formaldehyde is the first photosynthetic 

 product in the process of the production of carbohydrates from 

 water and carbon dioxide, the simple empirical equation for this 

 transformation would be 



H 2 O+C0 2 = CH 2 O+0 2 . 



